I'm in the same position regarding what system to get it on. Played CoD4 to death on 360. Pretty unlikely I'll get it for PS3 but recently got a high spec PC and most of my gaming buddies are likely to get it on PC. However, my "Live" friends (who aren't irl friends) will be sticking to the 360 and I had some good times with em. Tough one for me.
Regarding the Live experience, as far as I'm concered they should call the Xbox 360 the Xbox CoD
![Wink ;) ;)](/styles/default/xenforo/vbSmilies/Normal/wink.gif)
Party mode with private party mic chat + hitting tdm and search and destroy in mw2 will be multiplayer perfection.